Drug injectors such as insulin injectors have recently been developed for users having a need for periodic and continual drug injections. These drug injectors can be either implanted or externally located with respect to the user's body. A typical user of a drug dispensing injector or device would be a diabetic having a requirement for insulin injections whose amounts are related to the time, quantity, and type of food intake. Proper amounts of insulin for such a user are essential for health and even life itself, and it is therefore essential for the user to know the amount of insulin being dispensed as well as the amount of insulin remaining in a reservior, especially in the case of an implanted device. One method of determining the amount of fluid remaining in the reservoir is to note each drug injection time and the amount of drug to be dispensed. A running total of the amount of drug dispensed since the last reservoir refill can then be maintained. Such a method would be somewhat satisfactory if the dispensing apparatus always functioned as intended; however should it dispense slightly more fluid than intended due to a malfunction of some type, then the reservoir could run out of fluid at an unexpected and inconvenient time. A more positive method of determining the amount of fluid remaining within the reservoir would be to actually measure flow rate from the reservoir each time that the drug is dispensed. This flow rate and the time during which the flow rate existed would provide a precise indication of the amount of fluid removed from the reservoir. The present invention provides a means useful for determining the flow rate which in turn can be utilized to calculate the amount of fluid remaining in the reservoir.